The present invention relates to apparatus and method for electrical discharge machining using a rotating electrode for machining a desired profile into the surface of a conductive work piece.
Electrical discharge machining (EDM) is a well-known technique for the precision machining of hardened metal alloys and conductive ceramic composite materials (such as a cobalt-tungsten carbide among others) that are difficult or impossible to machine by more conventional methods. In EDM, the cutting tool is an electrode attached to a power supply. Material is gently removed from the work piece by striking a microscopically small spark between the forming tool and the electrically conductive work piece. To do this, the electrode is brought close to the surface of the work piece so an electric discharge or spark is generated across the gap between the electrode and the work piece. As the discharge enters the work piece, several molecules of the material of the work piece are ejected from the work piece surface. The material removed by the spark normally is flushed away by a dielectric liquid. In this fashion, the work piece can be machined to a desired shape.
One drawback of EDM is that the electrode erodes over time. This erosion of the electrode will change the dimensions of the eventual machined shape. Changing the electrode often or building the expected erosion into the starting electrode are ways to compensate for the erosion.
A common form of EDM and the process featuring the highest rate of material removal employs a wire electrode about 0.008 to 0.015 inches in diameter. The wire is connected to a spark generator and is moved in a direction normal to its longitudinal axis to cut into the work piece. In addition, the wire electrode is continuously indexed in the direction of its axis so as to present the same wire diameter to the work piece.
An EDM process using a wire electrode is capable of a dimensional precision of 0.001 to 0.00005 inches under ideal set up conditions. Flushing debris from about the electrode is accomplished by pumping the dielectric liquid directly into the cut. Reliance is on the axial movement of the wire to carry the flushing liquid into the cut for sweeping away the material ejected from the work piece.
Rotating the electrode also is known. For example, U.S. Pat. No. 4,001,538 discloses rotating the electrode about is longitudinal axis and discharging sparks from an end face of the electrode to smooth the surface of the work piece. Preferably the electrode is arranged with its rotational axis disposed at a slight angle from the vertical of between about 85xc2x0 to 89.40xc2x0 as measured from the plane surface of the work piece. The shape of the working end of the rotating and inclined solid cylindrical electrode is described as xe2x80x9cblunt conicalxe2x80x9d. The work piece is disposed in a bath of a dielectric liquid and the shape of the blunt conical end aids in flushing away debris and material removed from the work piece.
Another device using a rotating electrode for making threaded and non threaded bores is disclosed in U.S. Pat. No. 4,628,171 and a device for supplying an electric current to a rotating electrode is disclosed in U.S. Pat. No. 5,561,336
In EDM, controlling the frequency, voltage, current and wave shape of the spark discharge is known to control the surface finish imparted to the work piece. In this respect, there is a given amount of energy available per unit time depending upon the current applied to the electrode. For a greater number of sparks per second (and a reduction of the current and voltage of that spark) there will be less energy per spark so the amount of material removed per spark will be less. Thus, if a smooth surface is desired, a higher spark frequency (and reduced energy content of each spark) and longer work time is used as opposed to a rapid machining operation which uses a lower frequency to remove larger amounts of the work piece with each discharge.
EDM requires that the work piece be conductive. Accordingly, working certain ceramic materials including silicon carbide is particularly difficult in part because of the poor conductivity of these materials. Ceramics and silicon carbide have been used for surgical instruments such as scalpels and other cutting instruments and the sharpening of these materials to provide a cutting edge has been done by conventional means (such as Diamond Grinding) in part because of the conductivity issues. Conductivity problems may be overcome by known doping techniques to incorporate a conductive metal into the crystalline structure of the ceramic. While silicon carbide (SiC) can be made conductive, a drawback to machining this material by EDM is that SiC readily combines with free oxygen to form SiO2 on the surface of the work piece. Since SiO2 is an insulator, a layer of SiO2 only Angstroms thick, on the surface of the work piece will adversely effect the machining process. Any build up of SiO2 on the surface of the electrode also will interfere with the EDM process.
Accordingly, in view of the current state of the art there is a need for EDM apparatus and process for working harden metals and conductive ceramics to improve surface finish of the work piece. There also is a need for an EDM apparatus and method capable of dealing with the erosion of the electrode. There further is a need for an EDM apparatus and method for dealing with the build up of contaminants and electrical insulating process by-products on the electrode. This is particularly the case where the work piece is a silicon carbide and the contaminant includes SiO2.
In the present invention, the electrode is circular in cross section and may include such structures as a right cylinder, disk, wheel or conical body. The electrode also may be generally cylindrical with a desired profile that is symmetrical about the longitudinal axis of the electrode. The electrode is arranged to spin about its longitudinal axis and is arranged so that successive portions of the electrode periphery are carried into close proximity to the surface of the work piece. At the same time, portions of the electrode periphery on an opposite side are carried away from the surface of the work piece. For example, in the case of a cylindrical electrode, the axis of rotation may be oriented generally parallel to the surface of the work piece or within a few degrees of being parallel to the work piece surface so substantially the entire electrode is used in the EDM process. In the case of a conical electrode, the axis of rotation may be disposed at an angle to the surface of the work piece so the surface of the electrode is substantially parallel to the surface of the work piece.
The electrode is rotatably supported on high quality bearings and movement of the electrode is controlled such that the surface of the rotating electrode will approach the work piece surface in a predictable and controlled manner without plunging into the work piece. In this manner the surface of the work piece is machined by the adjacent portion of the electrode while a second portion of the electrode is remote from the work piece. In a preferred embodiment, the surface of the electrode is provided with a shape or contour that is machined into the surface of the work piece.
As the electrode spins about its longitudinal axis, the desired contour is formed in the work piece surface. A spinning electrode also allows for the use of an in situ dressing or sharpening of the electrode to compensate for the erosion of the electrode or to remove impurities that may build up on the electrode surface thereby extending the life of the electrode.
As the electrode spins in close proximity to the work piece surface, any suitable position monitoring system is used to monitor the position of the electrode for precise machining. Such a system may include for example a laser, optical system or video camera among others. The feed back from the position monitor and feedback from the electrical monitoring of the rate, duration, voltage and current of the pulse are all used as input parameters for computer assisted programming required for precision control of the electrode.
The spinning electrode may be used as the vehicle to carry a dielectric liquid between the electrode and the work piece to flush away the by-products of the machining operation by accelerating the flow of the dielectric liquid into the cutting area. Grooves can be provided in the surface of the electrode to enhance the pumping action forcing liquid into the cutting area. A pathway also may be provided through the electrode to facilitate the conduct of the dielectric liquid into the cutting area. In the case of a silicon carbide work piece, the dielectric liquid preferably is oil to retard the development of SiO2 on the surface of the work piece.
As the electrode spins one portion of the electrode surface is carried towards and into close proximity to the surface of the work piece while an opposite portion of the electrode surface moves away from the work piece. A suitable monitor can be arranged to monitor the profile of the electrode surface. Should the profile of the electrode change over time due to erosion or the build up of contaminants, the monitor sees the change. At such time as the change in the electrode surface become unacceptable, the electrode surface can be dressed to restore the desired profile. This is accomplished by operating a dressing tool to contact that portion of the spinning electrode surface that is spaced or remote from the work piece where the EDM is occurring.
The EDM process using a spinning electrode in accordance with the present invention preferably is used to provide a sharp edge, free of burrs or damage common to diamond or other abrasive grinding techniques. A preferred use is to sharpen ceramic or metal or harden metal blanks as may be used for surgical cutting instruments and the like. For example, blades can be formed of an electrically conductive ceramic, metal hardened metal, metal matrix composite, electrically conductive polymer or similar electrically conductive material that are near to a desired shape or profile of a finished blade. The spinning electrode, provided with a profile that is a positive (or reverse image) of the desired machined shape, then is used to perform the machining operation to provide the final shape.
Accordingly, the present invention may be characterized in one aspect thereof by an electro discharge machining (EDM) apparatus for removing material from a conductive work piece including:
a) a rotatable electrode having a generally circular cross section, the electrode being arranged for rotation about its longitudinal axis;
b) drive means for continuously rotating the electrode about its longitudinal axis to bring successive portions of the electrode surface into close proximity to the surface of the work piece such that electrical discharge machining of the work piece surface occurs at one portion of the electrode surface while another portion of the electrode outer surface is brought to a location remote from the work piece surface;
c) a dressing tool at the remote location for dressing the electrode surface;
d) a monitor for monitoring and indicating changes in the character and condition of the electrode surface from certain starting parameters; and
e) the dressing tool being operable in response to a change in the character and condition of the electrode surface for dressing the electrode surface and restoring the starting parameters.
In another aspect, the present invention may be characterized by an electric discharge machining process for removing material from a surface of a conductive work piece comprising:
a) providing a rotatable electrode having a generally circular cross section;
b) arranging the electrode for rotation about its longitudinal axis;
c) continuously rotating the electrode about its axis to bring successive portions of the electrode surface into close proximity to the surface of the work piece such that electrical discharge machining of the work piece surface occurs at one portion of the electrode surface while another portion of the electrode surface is brought to a location remote from the work piece surface;
d) monitoring changes in the character and condition of the electrode surface from certain starting parameters; and
e) dressing the portion of the electrode surface that is brought to said remote location by said rotating so as to restore the starting parameters.